Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and is not economical for formations containing small amounts of natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is converted with an oxidant such as water, molecular oxygen, or combination to form synthesis gas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch synthesis, carbon monoxide and hydrogen react over a catalyst to form organic molecules containing carbon and hydrogen, also known as hydrocarbons.
When hydrocarbons also contain oxygen, they are known as oxygenates. Hydrocarbons having carbons linked in a straight chain are known as linear hydrocarbons. Saturated hydrocarbons with single carbon-carbon bonds are called paraffins, and unsaturated hydrocarbons with double carbon-carbon bonds are known as olefins. Linear saturated hydrocarbons are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch synthesis is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch synthesis contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification), particularly nickel, iron, cobalt, and ruthenium. H. Schulz (Applied Catalysis A: General 1999, 186, p 3) gives an overview of trends in Fischer-Tropsch catalysis.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors in series or parallel. Commonly used reactors include packed bed (also termed fixed bed) reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. Because the Fischer-Tropsch synthesis is highly exothermic, proper temperature control within the reactor is a critical element. Packed bed reactors tend to have poor temperature control, compared to gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors,” “ebulliating bed reactor,” or “slurry bubble column reactors,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to products which are liquid and/or gaseous under reaction conditions. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor.
The products generated via the Fischer-Tropsch synthesis comprise a mixture of hydrocarbons containing from 1 to about 100 carbons or more. These hydrocarbon products are gaseous or liquid under reaction conditions. Under reaction conditions, liquid products comprise wax hydrocarbons, which are typically solid or semi-solid at standard conditions of temperature and pressure. Waxes can be further hydroprocessed for example in a hydrocracking unit to produce shorter, more branched hydrocarbons in the diesel range, thereby increasing the degree of isomerization of the diesel range hydrocarbons pool. Wax hydrocarbons are highly desirable for the production of fuel such as diesel, therefore good selectivity towards wax hydrocarbons is preferred, such that Fischer-Tropsch liquid product comprises a large amount of wax hydrocarbons.
Because of the continuous formation of products, it is necessary to continuously or intermittently remove them. The extraction of slurry from the reactor necessitates the separation of the Fischer-Tropsch liquid products (which include waxes) from the catalyst in order to substantially recycle the catalyst particles to the reactor, as well as to obtain clean liquid products. The substantial recycling of the catalyst back into the reactor is necessary to maintain catalyst inventory in the reactor in order to have a more cost effective process. The terms “wax hydrocarbon products”, “liquid products” and “wax” will be used interchangeably herethrough, as it is expected that the liquid hydrocarbon products comprises a majority of wax hydrocarbons. The terms “catalyst-wax separation” may be used for conciseness throughout the specification, and should by no mean limit the disclosure to the separation of catalyst from only wax. It implies separation of catalyst from liquid hydrocarbons (which might comprise wax).
Several techniques have been proposed for separating the catalyst from the liquid hydrocarbons, e.g., centrifuges, sintered metal filters, cross-flow filters, magnetic separators, gravitational settling, etc.
Filtration has proven to be one preferred catalyst-wax separation method used in the Fischer-Tropsch process. In conventional filtration techniques, a slurry is fed to the filtration unit that divides the slurry into a filtrate stream having a relatively low concentration of catalyst particles and a concentrated slurry stream having a relatively high concentration of catalyst particles. The concentrated slurry stream is then recycled to the reactor while the filtrate stream is processed to produce useful hydrocarbons. One of the major problems facing filtration systems is a decrease in filter efficiency over time (for example by plugging the filter substrate, by a decrease in permeability, and/or by a reduction in filtrate rate), which necessitates remedial action (such as backwashing of filter substrate, cleaning of filter substrate, and/or replacing of filter substrate) to resume a desirable filtration efficiency. Variation of filtration efficiency may result in unsteady production of liquid products, and the necessary maintenance is costly in operational down-time and/or additional capital costs.
Another known separation method employs a body force and is known as gravitational settling, also known as sedimentation, which seeks to take advantage of the differences in density between the solid particles and the liquid. Gravitational settling, as its name indicates, uses the action of gravitational force for suspended solids in a liquid to settle leaving an upper region depleted in particles and the lower region more concentrated in particles. Another separation method, which also employs a body force, is centrifugation, or centrifugal separation. Types of centrifugal devices include centrifuges and hydrocyclones. Centrifuges and hydrocyclones seek to enhance the body force (so that it is greater than the gravitational force) applied to the particles in order to accelerate the movement of the solid particles through the liquid and to promote separation from the liquid of small particles that otherwise would remain suspended due to the influence of Brownian forces. Similar to conventional filtration systems, most gravitational and centrifugal separation systems divide the slurry drawn from the reactor into liquid output streams, with one stream having a high concentration of catalyst particles and the other stream having a low concentration of catalyst particles. As a unit operation, gravitational settling (or sedimentation) offers low capital and operating costs; however it can be inefficient in capturing solid particles that have very low settling rates. On the other end, centrifugal units may be more costly to operate, but typically have an increased efficiency compared to that of gravitational settling on solid particle with very low settling rates.
Severe hydrodynamic conditions inside a commercial slurry bubble column reactor, coupled with the desired long lifetime of the catalytic material, typically result in catalyst attrition. As the catalyst breaks down over time, sub-particles of various sizes may be created, including very small particles known as “fines,” some of which may even be sub-micron in size. The presence of fines in the reactor tends to greatly reduce the effectiveness of the catalyst-wax separation system. With the presence of catalyst fines or sub-particles, conventional sedimentation may be ineffective in their separation. Typically, in the case of attrition-prone catalysts, sedimentation may be supplemented or complemented by centrifugation, filtration or ultra-filtration, which are much more costly and require high maintenance.
Despite its shortcomings of lower efficiency with small size particles, sedimentation is a commonly used process for the separation of suspended solids from a liquid as for example in the treatment of sewage, industrial wastewater, process water or drinking water. Wastewater treatment plants more particularly employ sedimentation for the collection of sediments, flocs, inorganic precipitates, and/or biological material, as sedimentation is cost effective and typically requires low maintenance. However, in many conventional sedimentation basins or clarifiers, many factors such as inlet and outlet turbulence, and inherent unequal flow distribution, can hinder the sedimentation, and therefore its efficacy.
Improvements in sedimentation technology for wastewater treatment has been possible by the use of inclined settlers, also called “lamellar” settlers or “supersettlers.” These improved settlers are operated under two fundamental principles: shortened settling path and laminar flow. The settling path is shortened in a settling vessel by the use of inclined surfaces. Because the sedimentation time is directly proportional to the vertical settling distance, a reduction in settling path in inclined settlers results in much reduced retention times by an order of magnitude or more below those in corresponding vertical settlers. These settlers can be composed of either long narrow tubes or channels inclined from the vertical or of a large tank containing closely spaced inclined plates. Thus, laminar flow can also be easily maintained in long narrow passages of small cross-sectional area that characterize these improved settlers. Water enters the inclined settler tubes and is directed upward through the tubes. Each tube functions as a shallow settling zone. Solids collect typically on the lower surfaces of the tubes and settle to the bottom of the basin.
The phenomenon of enhanced sedimentation in inclined channels was first described in Boycott, A. E. (1920); “Sedimentation of blood corpuscles,” Nature vol. 104, p. 532. A summary of early work on this subject is also described in Hill, W. D. (1974); “Boundary-enhanced Sedimentation due to Settling Convection,” PhD thesis, Carnegie Mellon University, Pittsburgh, Pa. The fundamental mechanisms and governing equations for sedimentation on inclined surfaces are further explored in Davis & Acrivos, (1985), Ann Rev Fluid Mech., vol. 17, pp. 91–118; Kapoor & Acrivos (1995) J. Fluid Mech. vol. 290, pp. 39–66; Tripathi & Acrivos (1996) International Journal of Multiphase Flow, vol. 22 (2), pp. 353–361.
The shallow depth sedimentation by inclined channels primarily used in wastewater plants achieves high performance at low cost. Thus, unlike the improvement in wastewater sedimentation techniques, the development of efficient, high-yield catalyst-wax separation systems has been one of the limitations on the commercialization of the Fischer-Tropsch slurry reactor system, due in part to the formation of fines or sub-particles from attrition-prone catalysts, which in time reduce the separation efficiency and stability.
Thus, there remains a need in the art for cost-effective methods and apparatus to efficiently remove valuable clean liquid hydrocarbons from a catalyst-containing slurry so that a substantial portion of the catalyst particles can be returned to the reactor. Therefore, the embodiments of the present invention are directed to methods and apparatus for recovering clean liquid products from a slurry, while substantially maintaining catalyst inventory in a slurry reactor, that seek to overcome certain of the limitations of the prior art.